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Find RPM of small fan using o-scope?

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David Turner

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This is my first post, so sorry if I'm noob-ish.

I'm trying to figure out how to find the RPM of a small DC brushless fan. I'd like to try to get it figured out with an oscilloscope. I'm having a hard time finding a good way to do this. Does anyone have any ideas on how I could do this? I'm also open to other ideas on how to figure this out. Thanks.
 

DerStrom8

Super Moderator
I would use an IR transmitter and detector, one on either side of the fan blades. Every time the blade passes between them, it will break the beam. You can monitor the signal on the receiver using the scope and measure the frequency, which you then divide by the number of fan blades. This would probably be the simplest approach.
 

picbits

Well-Known Member
You could also make a simple strobe light with an LED and a 555 timer. Mark a blade with a white spot, adjust the frequency to the strobe until the fan appears stationary then you can measure the frequency of the strobe and work it out from there :)
 

David Turner

New Member
So why an IR transmitter vs a photoresistor, or possibly using a hall effect sensor? I'm not disagreeing with you, I just would like to figure out the best option.
 

DerStrom8

Super Moderator
So why an IR transmitter vs a photoresistor, or possibly using a hall effect sensor? I'm not disagreeing with you, I just would like to figure out the best option.
A photoresistor works too instead of an IR receiver, just make sure it's not negatively affected by ambient light. A hall effect sensor would work except that you'd need to mount a magnet to one of the fan blades. This could slow it down a bit, so your reading wouldn't be accurate.
 

audioguru

Well-Known Member
Most Helpful Member
I use my hearing and the program Audacity to measure the RPM of my model airplane motors. I adjust the pitch of Audacity to match the pitch of the motor. Two blade propellers produce double the actual RPM and 3 blade propellers produce triple.
 

David Turner

New Member
A photoresistor works too instead of an IR receiver, just make sure it's not negatively affected by ambient light. A hall effect sensor would work except that you'd need to mount a magnet to one of the fan blades. This could slow it down a bit, so your reading wouldn't be accurate.
So you don't think the magnetic field produced by what inside the fan would be strong enough for the hall effect transistor to detect it? You're the third person to recommend putting a magnet on one of the blades.
 

DerStrom8

Super Moderator
So you don't think the magnetic field produced by what inside the fan would be strong enough for the hall effect transistor to detect it? You're the third person to recommend putting a magnet on one of the blades.
The magnetic field produced by the motor does not move in the way you'd want it to for a hall-effect sensor to pick it up correctly, without some sophisticated setup.

And to clarify, I am NOT recommending putting a magnet on one of the blades. I said that WOULDN'T be a good idea, because that would slow down the fan making the reading inaccurate. I recommend a light-based option (maybe a laser pointer aimed through the blades at a photoresistor).
 

David Turner

New Member
The magnetic field produced by the motor does not move in the way you'd want it to for a hall-effect sensor to pick it up correctly, without some sophisticated setup.

And to clarify, I am NOT recommending putting a magnet on one of the blades. I said that WOULDN'T be a good idea, because that would slow down the fan making the reading inaccurate. I recommend a light-based option (maybe a laser pointer aimed through the blades at a photoresistor).
Awesome. I'll play around with either the photoresitor or ir sensor and see what I come up with. Thanks for the help.
 

alec_t

Well-Known Member
Most Helpful Member
A simple method which I've used is to put a low value resistor in series with the fan and use the scope to monitor the voltage across it. At each commutation of the coils the current briefly drops. So the monitored voltage pulses occur at N times the rotation frequency, where N is the number of coils.
 

DerStrom8

Super Moderator
A simple method which I've used is to put a low value resistor in series with the fan and use the scope to monitor the voltage across it. At each commutation of the coils the current briefly drops. So the monitored voltage pulses occur at N times the rotation frequency, where N is the number of coils.
Isn't this only applicable to brushed DC motors? Not brushless?
 

dr pepper

Well-Known Member
Most Helpful Member
If you dont mind being invasive you could dig inside the fan motor, they are usually 2 phase motors, then solder on a wire to one of the coils and 'scope that, if there are 4 coils then the rpm would be the frequency x 60.
If the fan has 3 wires then the yellow is actually a tach signal.
 

alec_t

Well-Known Member
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atferrari

Well-Known Member
I tested a transistor (not sure if 2N2222 or 2N3055) which had the top filed on purpose, by exposing it to a ceiling light through a fan's blades. Good enough to calculate frequency: frequency of signal on the screen divided by No of blades.
 

dr pepper

Well-Known Member
Most Helpful Member
Connect a series current sense resistor inline with the fan, you'll see the commutations of the driver circuit & get rpm's from that.
 
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